Polypropylene foamed articles, with their excellent mechanical strength and heat resistance among synthetic resin foamed articles having light weight and thermal insulating properties, are important as housing and industrial materials. However, various technical problems arise when large amounts of polypropylene foamed articles are continuously produced, and there are demands for developing the continuous mass-production of polypropylene foamed articles.
The present invention provides a method for the continuous extrusion molding of polypropylene foamed articles. In particular, the present invention relates to a method for extrusion molding which is capable of producing uniform polypropylene foamed articles with low to high degrees of foaming and, particularly, those having large thickness.
An example of conventional methods for producing polypropylene foamed articles is a method in which a crosslinking auxiliary and a chemical foaming agent are mixed with polypropylene to form a sheet which is then crosslinked by ionizing radiation and foamed by heating. However, the maximum thickness of a product which can be obtained by this method is 20 mm because of problems due to the penetration of radiation and heating in subsequent processes. Therefore, the formation of a thick foamed article with this method requires the lamination of several such foamed articles. Another method is also known for producing polypropylene foamed articles in which polypropylene is impregnated with a volatile substance under pressure and is then foamed once to form preliminarily-foamed particles which are then foamed again in a pressure vapor mold. However, this method uses a batch system and thus has problems in that productivity is poor and large foamed articles cannot be easily produced.
The inventors have previously proposed a method which is capable of solving the above-described problems and can continuously produce polypropylene foamed articles (U.S. Pat. No. 4,552,708). In this method, the crosslinking of polypropylene and decomposition of a foaming agent are performed in a long-land die provided with a choking portion, followed by foaming. According to this method a polypropylene foamed article can be continuously provided. However, since the crosslinking agent and the foaming agent must be decomposed not in an extruder but in a subsequent long-land die, it is necessary to control the temperature of the extruder as low as possible. It is therefore necessary to select a polypropylene resin with as low a melting point as possible and with a high melt flow rate, and to uniformly mix a crosslinking agent and a foaming agent with the resin at as low a temperature as possible and extrude the thus-obtained mixture toward and inlet of the long-land die.
However, even if a polypropylene resin satisfying the above-described requirements alone is selected, cracks often emerge in the surface of the produced foamed article, and thus a foamed article with good appearance is not easily obtained. The inventors have achieved the present invention with a view to solving the above-described problems.